legal ground
Article 158 of the Civil Code
A civil juristic act may be conditional, except that it cannot be conditional according to its nature. A civil juristic act with effective conditions shall take effect when the conditions are fulfilled. A civil juristic act with conditions for dissolution shall be invalid when the conditions are met.
Article 160
A time limit may be attached to a civil juristic act, but it may not be attached according to its nature. A civil juristic act with an effective time limit shall take effect upon the expiration of the time limit. A civil juristic act with a termination period shall be invalid at the expiration of the termination period.